Reconstruction of Mediterranean flora, vegetation and climate for the last 23 million years based on an extensive pollen dataset

Abstract

In the last 46 years extensive effort has been devoted to the botanical identification of pollen grains from Neogene deposits. This robust approach is the only reliable way to reconstruct past flora and vegetation from pollen archives, additionally allowing fruitful comparisons with plant macroremains. Rich peri-Mediterranean fossil pollen data provide a comprehensive history of plant ecosystems during the last 23 million years. Successive disappearance of thermophilous-hygrophilous taxa has occurred. Severe changes affected the North Mediterranean vegetation which varied from prevalent forests to dominant open environments. Avicennia mangrove progressively perished from North to South and was replaced by Glyptostrobus swamps which in turn disappeared too in a diachronous way. Subtropical evergreen forests left room for Mediterranean sclerophyllous communities, and finally to Artemisia steppes which alternated with mesophilous forests during glacial-interglacial cycles. The South Mediterranean plant ecosystems contrasted with open subdesertic associations where Avicennia persisted up to the early Pliocene before the steppes invaded the lowlands. Neogene climate changes benefited the Mediterranean sclerophyllous plants that were already present in the early Miocene. Their subsequent fluctuations relate not only to temperature variations but maybe also to phases with high instability in seasonality and low variability in warmth. When present, Microtropis fallax is a serious candidate for identifying the primary Mediterranean sclerophyllous assemblages. Interpretation and climate quantification of the pollen data show a well-marked latitudinal contrast in the Mediterranean area both in temperature and humidity/ dryness. The thermic latitudinal gradient is characterized by a significant increase in the late Miocene towards its modern value. In the late Pliocene, climate evolved from overall warm temperatures and dry seasons to cooler winters and dry summers. General dryness and colder conditions developed during glacial periods, alternating with moister and warmer conditions during interglacials. Present refuges of Zelkova and Pterocarya are discussed. Desiccation of the Mediterranean led subdesertic plants and cedar to migrate in opposite ways. The present distribution of Cedrus illustrates its weakness against coolings although it seems to have been saved by geodynamic events.